In some processes it is necessary to provide a valve to choke fluid flow within a range between one extreme of a high flow rate with a low pressure drop across the valve to the other extreme of a low rate with a high pressure drop. One example of such a process is secondary recovery of oil from an arctic oil field. A high pressure fluid, usually water, is pumped down one or several wells to drive the oil to one or several production wells where the oil is recovered and lifted to the surface.
To practice the above method of secondary recovery, the water should be pumped into the well at a high flow rate to fill the well and formation as quickly as possible. In the event of a power failure or other operational shut-down, it is necessary to throttle fluid flow to a lower flow rate due to the limited capacity of the standby pump to maintain at least some flow to prevent the water from freezing in the wells or the supply lines. To control secondary recovery, the flow of water into the formation may also have to be controlled to a rate between the high and low flow rates.
The rangeability of prior art choke valves, i.e., the ability to choke in a range from a high flow rate with a low pressure drop across the valve to a low flow rate with a high pressure drop, has been found to be insufficient for certain processes including the secondary recovery method set forth above. Typically the rangeability of known choke valves, expressed in the ratio of the flow coefficient (hereinafter referred to simply as C.sub.v) of the valve at the high flow rate condition to C.sub.v at the low flow rate condition, has fallen between 30:1 to 60:1. C.sub.v is a function of the flow area at the throat of the choke valve as determined by testing. In the secondary recovery process described above, the water injection control valve may be required to have a rangeability of 340:1. Accordingly it can be understood that prior art choke valves do not have the rangeability demanded for some applications, in particular, water injection for secondary recovery.
While it has been known to use choke valves having insufficient rangeability, such use results in several problems. Since prior art choke valves have a single throat and plug to choke flow, the required variation of flow area to achieve the desired rangeability may result in high fluid velocities. This, in turn, results in cavitation. Cavitation usually occurs at the low flow, high pressure drop end of the control range and produces valve noise. Along with creating noise, cavitation also results in damage to the valve components such as the valve throat and the plug. Accordingly, there is a need for a valve having sufficient rangeability to be suitable for applications such as water injection secondary recovery without causing valve noise and cavitation.